In navigation of vessels, there is a dangerous area where a collision or an accident avoidance behavior occurs more frequently than in other areas. Therefore, there is a technique that performs risk determination for navigation of a vessel based on a relative distance between vessels.
However, even by the risk determination for the navigation of the vessel based on the relative distance between the vessels, it is difficult to detect a dangerous site having a high collision risk with high accuracy in some cases. For example, in navigation in a port, entering to a port facility and continuous traffic on a sea route in the port are present in a mixed manner. Therefore, in the risk determination based on the relative distance between the vessels, a course change for entering from the sea route to a destination pier may be erroneously detected as an avoidance behavior.
Meanwhile, the dangerous site is present not only in a place where a sea route is specified, such as a port, but also in a place where no sea route is specified. Tracks of vessels in a sea area where no sea route is specified include tracks of vessels respectively heading to various destinations. Therefore, when a certain vessel has turned, this turning is not always an avoidance behavior, and it is difficult to determine from a track itself whether the vessel takes an accident avoidance behavior.